Combustion Instability Analysis for Gas Turbine Combustors Using Comprehensive Combustion Model and FTF-based Approach

Abstract

Combustion instability in gas turbine combustors is caused by a resonant feedback among the flame zones, acoustic waves and the flow dynamics. The prediction of combustion instability phenomenon has long been a greatly challenging topic because all mechanisms leading to an unstable behavior are not fully understood. The approaches for combustion instability analysis have been largely classified by computational fluid dynamic (CFD) models, low-order flame models, and finite-element based thermos-acoustic analysis models. The state-of-art approaches for combustion instability analysis have their own pros and cons in term of accuracy and computational efficiency. Thus, this study is aiming at developing the optimum approach between accuracy and computational efficiency. In this aspect, the present approach is based on a simplified assumption that the relationship between the flame and acoustics are mainly affected by fluctuation from nozzle to flame zone. To maintain the computationally manageable procedure, the Flame Transfer Functions (FTF) are obtained by using the steady CFD results. To numerically analyze the thermos-acoustic instability, these calculated flame transfer functions are utilized to solve the Helmholtz equation in context with the finite element method (FEM). First, this study analyzed the role of each of the coefficients in the FTF that enters the source term of the Helmholtz equation through a simple calculation. As the second step, it is verified that this procedure is capable of improving the accuracy in comparison with the measured FTF data and the numerical results by using the predicted convection time in a source term of the Helmholtz equation. Next, this work has made the comparison between by LES results and solutions of Helmholtz equation using FTF calculated by the steady RANS results in the partially premixed combustor. The discussions are also made for the computation time and verified the effectiveness of the proposed approach. Moreover, the numerical simulation has been carried out for three-dimensional partially premixed PRECCINSTA combustor with the gas-turbine like configuration. Next, numerical simulation is carried out for a single nozzle of a gas turbine combustor with a real-world industry setting. Using the water cooled experimental data, it was found that the variation of the material properties outside the combustor possibly affects the acoustic field while it does not sensitively modify the flame characteristics. Thus, the variation effects of the material properties are considered only for the the acoustic field. To minimize the computational burden, two actual can-annular gas turbine combustors are tested, compared, and verified in this work. Based on computational results obtained in this study, the precise discussions are made for the capability and limitations of the present approaches as well as the future modeling direction especially for the complex industrial-scale gas turbine combustors.